Current-polarized ground-directional relay for parallel lines



June 17, 1941. LENEHAN 2,246,310

CURRENT-POLARIZED GROUND-DIRECTIONAL RELAY FOR PARA LEL LINES Filed April 18, 1939 INVENTOR Bernarafl ene ha' ATTORNEY WITNESSES:

Patented June 17, 1941 CURRENT-POLARIZED GROUND-DIREC- TIONAL RELAY FOR PARALLEL LINES Bernard E. Lenehan, Bloomfield, N. J., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application April 18, 1939, Serial No. 268,531

3 Claims.

My invention relates to protective relaying systems for transmission lines, and it has particular relation to the ground-directional relay elements for systems involving two grounded three-phase transmission lines which are parallel to each other for some distance, so as to have a material amount of zero-phase-sequence coupling, the two lines having relaying points at a common substation, and both being grounded at said substation.

An object of my invention is to provide novel means for preventing a faulty response of a current-polarized ground-directional relay, for such a system, in response to inductively induced ground currents, which flow in on the line and down in the ground-connection, and which produce the same directional efiect, in the relay, as actual ground-currents which flow out on the line and up in the ground-connection.

A more specific object of my invention is to polarize the ground-directional element with a current which is responsive to the sum of both of the currents in the ground-connections for both of the aforementioned mutually coupled lines, thus always obtaining a correct polarizing direction because the actual ground-currents in the faulted line are always larger than the induced ground-currents in the clear line.

With the foregoing and other objects in view, my invention consists in the systems, combinations, methods and apparatus hereinafter described and claimed, and illustrated in the accompanying drawing, the single figure of which is a simplified diagrammatic view of circuits and apparatus embodying my invention in a preferred form.

In the drawing, my invention is illustrated as being embodied in a three-phase transmission system involving two separate three-phase transmission lines LI and L2, which extend, for some distance, parallel to each other, in close juxtaposition, as indicated, so as to have a material zero-phase-sequence coupling. The two lines LI and L2 each have an end in a common substation, as illustrated, where the line Ll is illustrated as being connected, through a circuit breaker CBI and delta-star step-up power transformers PTI, to a generator or generators GI; while the line L2 is similarly connected to a generator or generators G2 through a circuit breaker CB2 and power transformers PT2. The star windings of the respective power transformers PTI and PT2 are the high-voltage windings which are connected to the respective lines Li and L2, and these star-connected transformer windings have their neutral circuits grounded, as indicated, to provide grounding connections for the respective lines Li and L2.

The circuit breaker CBI of the line Ll is provided with a trip coil TCI which is energized in response to a ground-directional element DOI and an over-current ground-relay IOI. The ground-directional element DOI is provided with two sets of terminals, which may be referred to as the directional terminals and the polarizing terminals, respectively, so that the directionalelement compares the relative directions of the currents in the two terminals. The operating coil of the ground overcurrent relay IOI and the directional terminals of the ground-directional relay DOI are energized with the neutral current of a bank of star-connected line-current transformers CTI, which are illustrated as also supplying current to line-current-responsive devices Al, BI and CI.

The circuit breaker CB2 of the second line L2 is likewise provided with a trip coil TC2 which is controlled by a ground overcurrent relay I02 and a ground directional relay D02, which are similarly energized from line-cturent transformers 0T2, which also supply phase-current-responsive relaying loads A2, B2 and C2.

In accordance with my invention, the polarizing terminals of the two ground-directional elements DOI and D02 are energized in accordance with the sum of the currents in the grounded neutral points of the star-connected windings of the respective power transformers PTI and PT2. This energization is obtained with the aid of auxiliary neutral-current transformers NT! and NT2, respectively, connected in the grounded neutrals of the two banks of power transformers PTI and PTZ, the secondary coils of these neutral current-transformers NT! and NT2 being connected in parallel, in order to obtain the sum of the grounded-neutral currents, in accordance with my invention.

In operation, let it be assumed that there is ground-fault somewhere out on the first line Ll, as indicated at X in the drawing. This will produce zero-phase-sequence line-currents I01, which may be regarded as flowing out, on the line LI, and up, in the ground-connection of the power transformers PTI. By reason of the zero-sequence coupling between the two lines, the zero-phase-sequenoe ground-fault current I01 in the faulted line Ll will induce, in the other line L2, a somewhat smaller, oppositely flowing zero-phase-sequence current I02, which flows in, on the line L2, and down, in the grounded neutral connection of the power transformers PTZ.

If the ground-directional relay D02 of the second line L2 were polarized simply with the current in the grounded neutral of the power transformers PTZ, as was heretofore common in the art, thispdirectional element would respond to the above-mentioned induced currents, the same as if the ground-fault were on the line-section side of the relaying point, because the directional-element cannot distinguishbetween cur-.

rents flowing in and down, and currents flowing out and up.

In accordance with my invention, however,

the polarizing terminals of eitherone or both of" the directional elements DOI and D02 are energized in accordance with the sum of the two grounded-neutral currents, so that they'will'be responsive to an upwardly flowing groundedneutral resultant whether the ground-fault is on the line Ll or thelineLZ. In this manner, a directionalresponse is obtained only in response to actual" residualline-currents which are flowing in such a directionas; to indicate that the ground fault is on the line-current'side of the relaying point, the directional elements being quite unresponsive,to the oppositely flowing induced residual line-currents.

While I have illustrated my invention in a preferred form of embodiment, it will be obvious that manychanges, such as those which have been suggested during the course of the description, and other changes not alluded to, may be made; by. those skilled in the art, without departingfromthe essential features of my invention. I desire, therefore, that. the appended claims shall be accorded the broadest construction consistentv withntheir language and the prior art.

I claim as my invention:

1. In combination, two grounded polyphase transmission lines having a material amount of zero-sequencecoupling therebetween, and having relaying points at a common relaying station, ground directional relaying-means, at said station, for at least one of said lines, said directional relaying-means having two sets of-terminals for comparing the relative directions of the respective currents therein, a set of star-cone in every case, no 'matternected line-current transformers associated with said one of the lines for energizing one of said sets of terminals responsively to the neutral current therein, means for-so energizing the other set of terminals as to be additively responsive to the sum of zero-sequence components in the two lines, and-means responsive 'to the condition of said relaying-meanswhen a ground fault oc-. ours on said one of said lines for interrupting service on said one of said lines.

2. Incombination, two polyphase transmission lines having a material amount of zero-sequence coupling therebetween, and having relaying points at a common relaying station, separate grounded star-connected means for the two lines at said station, ground directional relayingmeans, at said station, for at least one of said lines, said directional relaying-means having two sets of terminals for comparing the relative directions of the respective currents therein, a setof star-conneoted line-current transformers associated with said one of the lines for energizing one of saidsets of terminals responsively to the neutral current-thereinymeans for energizing the other setof terminals additively in response to the ground-currents in the grounded star-connected means on both lines, and means responsive to said relaying-means for interrupting service on said one of said lines.

3. Means for determining on which side of a grounding point, in-a protectedv grounded polyphase transmission line, a ground-fault may be located, under circumstances where saidline is, atleast to a material extent, paralleled by an other grounded polyphase transmission line having a grounding point at the same location,-said means comprisingthe combination, with said lines, of means for obtaining a response to the zero-phaseesequence component of, the linecurrent inlthe-protectedline, means for deriving a reference-current. comprising at least two additively combined components responsive, respectively, to the ground-currents in the two grounding points, and 7 means for comparing the relative direction of said zero-phase-sequence component with respect to saidv referencecurrent.

BERNARD'E. LENEHAN. 

